1. Field of the Invention
This invention relates to a two-needle sewing machine which performs two stitching operations with two sewing needles at the same time, and more particularly to a two-needle sewing machine in which each of the sewing needles is provided with a thread cutting mechanism.
2. Related Art
FIG. 11 shows a two-needle sewing machine 1. The sewing machine 1 comprises: a sewing machine body 3 mounted on the upper surface of an operating stand 2; a motor 4 and a sewing machine control device 5 which are mounted below the operating stand 2; and an operating pedal 6 set near the floor. When the operating pedal 6 is stepped on, the motor 4 is driven under control of the sewing machine control device 5. The rotation of the motor 4 is transmitted through an endless belt to a sewing machine pulley 8 which is fixedly mounted on the spindle of the sewing machine body 3. The torque of the spindle rotating together with the pulley 8 is transmitted through a cam machine to a cloth feeding mechanism and a needle driving mechanism in the sewing machine body 3, so that two stitching operations are carried out with two needles 9A and 9B in a parallel mode (at the same time) while a material such as a fabric to be sewn is being fed.
The sewing machine body 3 includes a table section 3a, in which, as shown in FIG. 12, a pair of thread cutting mechanisms 11A and 11B are provided below the sewing needles 9A and 9B, respectively. A lock-stitching shuttle race (not shown) is provided below each of the thread cutting mechanism 11A and 11B. The thread cutting mechanism 11A has a stationary knife 12A and a movable knife 13A. Similarly, the thread cutting mechanism 11B has a stationary knife 12B and a movable knife 13B. The stationary knives 12A and 12B are secured horizontal from stationary parts 15 of the sewing machine body 3, respectively, while the movable knives 13A and 13B are coupled to the upper end portions of rotary shafts 16, respectively, in such a manner that they are horizontally swingable. In order to drive the thread cutting mechanisms 11A and 11B, cylinders 14A and 14B are provided, as thread cutting actuators, below the thread cutting mechanisms 11A and 11B. In the cylinders 14A and 14B, cylinder bodies 17A and 17B are fixed in the sewing machine body 3 in such a manner that they are extended horizontally, and the end portions of cylinder rods 18A and 18B are coupled through hinge mechanisms 19A and 19B to the lower ends of the rotary shafts 16, respectively. With the thread cutting mechanisms, thread cutting operations are carried out as follows: The cylinders 14A and 14B are driven to swing the movable knives 13A and 13B, so that the threads are positioned in place with the end portions of the movable knives 13A and 13B, and then the threads thus positioned are cut with the movable knives 13A and 13B in cooperation with the stationary knives 12A and 12B.
FIG. 13 is a block diagram showing a control system provided for a conventional two-needle sewing machine, and FIG. 14 is a time chart showing the timing of the driving of the thread cutting actuators (i.e., the cylinders 14A and 14B).
In FIG. 13, reference numeral 7 designates a thread cutting control circuit provided in the sewing machine control device 5. The circuit 7 comprises: a central processing unit (CPU) 20; and a pedal sensor 21, a cycle origin detector 22, a needle position detector 23 and a motor speed detector 24 which are connected to the input section of the CPU 20; and a motor driver circuit 25 and a thread cutting drive 26 which are connected to the output section of the CPU 20. The CPU 20 applies a motor drive instruction signal to the motor driver circuit according to detection signals from the pedal sensor 21 and the detectors 22 through 24 and to data stored in its memory, to rotate the motor 4 at a predetermined speed, and increases the number of stitches by one whenever the needle position detector 23 detects the needle upper end. When the number of stitches reaches a predetermined value corresponding to one sewing cycle, the CPU 20 applies a motor stop instruction signal to the motor driver circuit 25. In addition, upon detection of a needle up signal a predetermined number of stitches (for instance one stitch) before the sewing needles 9A and 9B are stopped, the CPU 20 applies a thread cutting instruction signal to the thread cutting driver circuit 26.
In response to the thread cutting instruction signal, the thread cutting driver circuit 26 applies thread cutting drive signals S to the two thread cutting cylinders 14A and 14B at the same time, thereby causing the latter 14A and 14B to perform the thread cutting operations.
The above-described operations are repeatedly carried out to repeatedly perform the stitching operation of one and the same pattern.
The above-described stitching operation is carried out for instance in the case where belt loops are sewed to trousers one after another. In this case, both end portions of a belt loop are sewed with the two sewing needles 9A and 9B at the same time. That is, a stitching operation of a pattern corresponding to a sewing cycle from the start to the end of the sewing operation, is carried out for each belt loop.
The above-described conventional thread cutting control circuit 7 is designed so that the two thread cutting cylinders 14A and 14B are driven by the one thread cutting driver circuit 26. Hence, the two thread cutting drive signals S1 and S2 output by the thread cutting driver circuit 26 are turned on and off with the same timing. This feature provides the following difficulty: In the case where thread cutting cylinders (14A and 14B having different response times (t1 and t2) from each other must be employed because of the mechanical space of the sewing machine body 3, or in the case where, although the thread cutting cylinders have equal performances, they become different in response time (t1 and t2) because of a difference in mechanical load between the thread cutting mechanisms 11A and 11B, the two thread cutting cylinders 14A and 14B are started at different time instants. That is, it is impossible to start the two thread cutting mechanisms 11A and 11B at the same time.
In the case of FIG. 14, one of the thread cutting mechanisms, namely, the thread cutting mechanism 11A, starts the thread positioning operation in the response time t1 after the thread cutting drive signal S1 is raised to an "on" level, and starts the thread cutting operation in the response time t3 after the thread cutting drive signal S1 is set to an "off" level. However, the other thread cutting mechanism 11B starts the thread positioning operation in the response time t2 (t1&lt;t2) after the thread cutting drive signal S2 is raised to an "on" level, and starts the thread cutting operation in the response time t4 after the thread cutting drive signal is set to an "off" level. That is, the thread cutting operation by the other thread cutting mechanism 11B is not started until the lapse of the period of time (t2-t1) after the one thread cutting mechanism 11A starts the thread cutting operation.
In FIGS. 14 and 15, reference character b designates the range of speeds of the spindle in which the thread positioning operation can be stably achieved with respect to the operations of the sewing needles 9A and 9B. Where the thread positioning operation is achieved with the speed of the spindle in the range b, as in the case of FIG. 14, the two thread cutting mechanism 11A and 11B can start the thread cutting operations correctly. However, if the sewing machine which is going to be stopped has a high speed, the thread positioning operation of the thread cutting mechanism 11B, which has a slower "on" response (or which, when the thread cutting drive signal is raised to "on" level, responds later), may not be achieved within the range b as shown in FIG. 15. This fact increases the probability of a failure in cutting of the thread, and adversely affecting the stitching operation.
Hence, the two-needle sewing machine 1 must be operated at sufficiently low speed for the response of the thread cutting cylinder which has a slower "on" response (namely, the thread cutting cylinder 14B) to achieve a speed within the range b with which the thread positioning operation can be stably achieved. Therefore, it is not permitted to operate the sewing machine at high speed until just before the sewing machine is stopped. Hence, in the case where a repeated stitching operation is performed, the sewing cycle time cannot be shorten, which makes it difficult to improve the sewing efficiency.